Hi Enrico,
thank you for your post! From Rebelle 2 the impasto effect has been added to the UI - more specifically you could set the "Impasto Depth" in Select Canvas window (after you clicked on the canvas).
In Rebelle 3 we moved it to the "Visual settings" panel - you can open this panel from Layers panel or via Window menu > Visual Settings. Under "Acrylics" tab, you can set the impasto depth from 0 (no effect) to 10 (strong effect):
By default it is set now to 5, that is why it may appear a bit weak to you.
Let me know whether this helped.
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